1. Field of the Invention
This invention relates to lens surfacing apparatus and has particular reference to improvements in toric surface generators.
2. Discussion of the Prior Art
A toric surface is a surface of compound curvature frequently used ophthalmically for the correction of astigmatism. By compound curvature it is meant that the radius of curvature in one meridian is different than the radius of curvature in a second orthogonal meridian.
Because of the large commercial and practical importance of toric surfaces, a number of techniques have developed for their production. An early technique involved the use of preformed tools each having the shape of a particular toric curve desired on a lens, i.e. a mirror image of the desired lens surface shape. This preformed tool was abraded against the lens surface in conjunction with abrasive slurries in such a way that gradually the lens assumed the shape of the tool. This produced reasonably accurate toric surfaces. However, because of rapid tool wear and the vast inventory of preformed tools needed to satisfy the hundreds of combinations of the two meridianal radii of curvature encountered in the field, preformed tools have been largely replaced by a rotating cupped or ring tool. This tool typically has an annular working edge which abrades the workpiece, be it glass or plastic. The toric surface is achieved by having the radius through which the ring tool is swung be substantially the same as one of the desired radii of lens surface curvature. The second radius of curvature in a meridian at right angles to the first is achieved by a tilt of the ring tool so that the profile of the tool assumes approximately the curvature of the second radius. The universal nature of being able to modify independently both the radius of swing and the angle of tool tilt eliminates the need for large tool inventory. Unfortunately, in the process of using the angle of tilt to modify one effective tool cutting radius, an eliptical error is introduced so that the lens surface formed is not a true toroid. This eliptical error is in most cases very significant. It requires subsequent surface grinding to eliminate if one is to achieve optimum lens performance.
A number of attempts have been made to overcome the problems associated with undesirable eliptical error. In one case, the eliptical error was minimized by moving the tool relative to the lens in a series of complex motions which necessitated correspondingly complex and expensive machinery not suitable for use in custom laboratory operations. More recent attempts used a grinding tool which was swung through one of the desired radii of curvature with its own radius of curvature being that of the second radius of curvature desired on the lens. This, however, necessitates a separate tool for each second radius of curvature and hence, still requires costly tool inventory. Such a need for large tool inventory has, however, been reduced by still using the aforementioned cupped or ring tool which is swung through one radius with the orthogonal tool profile assuming the curve along a second meridian which is simultaneously modified with an oscillating motion of the lens relative to the tool. Although theoretically capable of producing desired surface curves, this scheme is extremely cumbersome and difficult to implement and lacks the rigidity necessary for successful use.
Examples of the above toric generating schemes and apparatuses can be found in U.S. Pat. Nos. 2,548,418; 2,633,675; 2,724,218; 3,117,396; 3,492,764 and 3,624,969.
With a view to overcoming the above and related shortcomings of the prior art, it is an object of this invention to simplify the manufacture of true toric surfaces and more particularly to avoid eliptical error defects in ophthalmic lens surfaces intended for the correction of astigmatism.
Another object is to accomplish the foregoing with minimal capital equipment expenditure.
Still another object is to provide an apparatus for generating true toric lens surfaces, a single tool universality to the production of various preselected combinations of spherical and cylinder curvatures.
A further object is to provide toric surface generating apparatus of minimal mechanical complication and costliness and requiring no special skills to operate.
Other objects and advantages of the invention will become apparent from the following description.